It is well-known that a synthetic polymer film, for example, polyester film, can be wound on a core to form a film roll by means of a center-winding method or a surface-winding method.
In the center-winding method, a core is put on a winding drum which is driven at a predetermined speed and the film is wound on the core by driving the winding drum. In this method, usually, nothing comes into contact with the surface of the film which is being wound. Sometimes a touch roll, which is rotatable around a center shaft thereof, is placed on the surface of the film on the winding drum so as to press the film.
In the surface-winding method, a rotation drum is driven at a predetermined speed and a winding drum, which is rotatable around a center shaft thereof and which is covered with a core, is placed on the rotation drum in such a manner that the peripheral surface of the core on the winding drum comes into contact with the peripheral surface of the rotation drum and the core on the winding drum is rotated by the rotation drum due to the friction created between the core surface or the resultant film roll surface and the rotation drum surface.
In another method, the film may be wound to form a film roll by means of the surface-center-winding method, which has the advantages of both the surface-winding method and the center-winding method. In the surface-center-winding method, a winding drum is directly driven in the same manner as that of the center-winding method, and a touch roll is placed on the film roll formed on the winding roll so as to press the surface of the film roll. The touch roll is rotatable around the center shaft thereof and is driven due to the friction created between the peripheral surface of the touch roll and the surface of the film roll on the winding drum. The touch roll is effective for stabilizing the winding procedure of the film due to the pressing action thereof and for causing the resulting film roll to exhibit a satisfactory configuration.
When a polyester film is wound by means of the above-mentioned winding methods, the film is wound together with air accompanying the film surface. That is, the resultant film roll contains layers of air formed between the layers of film. Therefore, it is important to control the amount of air contained in the film roll to within an adequate range. Generally, if the amount of air is excessively large, the friction between the layers of film is significantly reduced. The reduced friction allows the film layers to easily move in a zigzag lateral direction and, therefore, the side faces of the resultant film roll become rough. This phenomenon is so-called "side face deviation".
If the film in the film roll shrinks due to the inherent shrinking property thereof, the winding is spontaneously compressed so that the air contained between the layers of film is removed through the side surfaces of the film roll and the inner portion of the film roll is deformed due to the shrinking force of the outer portion of the film roll.
If the amount of air contained in the film roll is excessively small and if the thickness of the film is uneven in the lateral direction thereof, a thick portion of the film forms a belt-shaped protuberance on the periphery of the resultant film roll. This phenomenon results in roughening of the film surface and causes the film surface to exhibit decreased physical properties.
Accordingly, in the winding procedure for producing a film roll, it is important that the amount of air in the air layers formed between the layers of film superimposed on each other be adequate.
When the film roll is formed by means of the center-winding method, the air in the air layers is often present in an excessively large amount. In this case, the amount of air in the air layers can be controlled merely by controlling the amount of winding tension to be applied to the film. If the amount of winding tension applied to the film is too small, it is impossible to reduce the amount of air in the air layer. Also, if the amount of winding tension is excessively large, the film is wrinkled just before it is introduced into the film roll and therefore wrinkled film is introduced into the film roll.
In the surface-winding method, it is necessary to keep the pressure between the peripheral surfaces of the rotation drum and the film roll constant so as to stabilize the rotation of the rotation drum. However, when the diameter of the resultant film roll becomes large, it is difficult to keep the pressure applied the surface of the film roll constant because of deviation in load on the rotation drum, which deviation in load is derived from a disturbance which cannot be controlled. Generally, the amount of air in the air layers formed between the layers of film is variable depending on the pressure applied to the surface of the film roll. Accordingly, in the case of the surface-winding method, it is very difficult to control the amount of air in the air layers.
However, in the case of the surface-center-winding method wherein the film roll is formed around a winding roll and the surface of the film roll is pressed with a touch roll, the amount of air in the air layers formed between the wound film layers can be controlled by controlling the surface property of the touch roll and/or the pressure derived from the touch roll. This is a benefit of the surface-center-winding method.
Due to the above-mentioned benefit, the surface-center-winding method is mostly utilized in a slit-winding apparatus by which a film is slit to a predetermined width and then is wound up to form a film roll.
In the recent winding-up technique for a film roll, it is important to control the amount of air in the air layers formed between the wound film layers. That is, the amount of air in the air layers is adjusted to a desired number on the basis of the brand of the film roll. Also, in the recently developed technology for producing polyester film, substantially no inequality in thickness is present in the polyester film. Accordingly, in the recent echnology for forming a film roll by using a slit-winding apparatus, it is important that the resultant film roll exhibit a satisfactory stability in configuration over a long period of storage. The stability in configuration of the film roll can be ensured by controlling the amount of air in the air layers in the film roll.
Usually, when a polyester film roll is stored over a long period of time, moisture in the atmosphere is gradually absorbed by the film roll through the peripheral surface and side surfaces of the film roll. The absoption of moisture results in the slight elongation of the film. This elongation causes the formation of wrinkles in the wound film layers in the film roll. Usually, wrinkles form a few days after the film roll is formed. Also, wrinkles frequently form in the case of a film roll consisting of a polyester film having an excellent smoothness of the surface thereof.
In the case of the conventional polyester film roll consisting of a polyester film having a thickness of 25 microns or less, especially 16 microns or less, and a centerline average surface roughness of 0.05 microns or less, it is very difficult to prevent the formation of wrinkles on the film by controlling the winding procedure wherein the air layers are necessarily formed between the wound film layers. That is, even when the winding tension to be applied to the film or the pressure to be created by the touch roll is strictly controlled in consideration of the brand of film to be wound up, it is difficult to prevent the formation of wrinkles because the manner of formation of wrinkles is unknown. Even if a film roll having no wrinkles is obtained, it is impossible to reproduce a film roll having no wrinkles since it is not clear how to prevent the formation of wrinkles. Accordingly, when the conventional film roll is shipped, it is impossible to guarantee that no wrinkles will form over a long period of storage.
As stated above, in the conventional technology, it is impossible to provide a polyester film roll which is completely free from the formation of wrinkles over a long period of storage. That is, when the conventional polyester film roll is exposed to the ambient air, wrinkles form in the film roll in approximately one week. In some cases, wrinkles form in approximately half of the outer portion of the film roll in a radial direction. The portion of the film having wrinkles is useless and, therefore, the film roll must be unwound and the portion of the film having wrinkles must be removed. Accordingly, the film roll having wrinkles has a poor commercial value.
Therefore, the inventors of the present invention considered that a polyester film roll which is free from the formation of wrinkles should be produced in consideration of the surface condition of the polyester film and the roll hardness of the film roll. On the basis of the above-mentioned consideration, the film roll of the present invention was created.
As stated hereinbefore, the touch roll used in the surface-center-winding method is effective for controlling the pressure to be applied to the surface of the film roll formed around the winding drum. Control of the pressure is effective for controlling the amount of air in the air layers in the film roll. The touch roll is usually coated with a rubber material, for example, NBR, EPT, polyurethane rubber or silicone rubber. Hitherto, the type of rubber material was selected in consideration of the purpose and manner of using the touch roll. However, the criterion for adjusting the hardness of the rubber material to be used to form the peripheral surface layer of the touch roll was not clear. Accordingly, the hardness of the rubber material for producing the touch roll was selected in consideration of the molding property and the resistance to scratching of the rubber material.
Recently, the polyester film surface has been designed in various ways, and in some type of films the winding conditions for the films is variable depending on the hardness and surface property of the touch roll. Therefore, the configuration of the resultant film roll depends on the hardness and surface property of the touch roll.
The inventors of the present invention considered that the hardness and surface condition of the touch roll should be adjusted in consideration of the surface condition of the film roll to be produced. On the basis of the above-mentioned consideration, the method of the present invention was completed.